1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device. More specifically, the invention relates to a nonvolatile semiconductor memory device including memory cell units each having a given number of memory cell transistors arranged in series in a column direction, such as a NAND flash memory (EEPROM: electrically erasable and programmable read only memory).
2. Description of the Related Art
As is known, a nonvolatile semiconductor memory device such as a NAND flash memory is useful in allowing data to be rewritten (written and erased) electrically though it is nonvolatile. Further, the memory device can increase in packing density and is decreasing in area (a transistor is decreasing in size) year by year.
Under the above circumstances, a nonvolatile semiconductor memory device has been proposed which is capable of greatly decreasing the size of a memory cell transistor whose gate electrode has a trench structure (refer to Jpn. Pat. Appln. KOKAI Publication No. 2001-274366, for example). In the proposed device, a memory cell transistor having a channel component in a vertical direction is formed to take measures against short-channel effects (SCE) due to the scaling of gate length.
However, the above prior art nonvolatile semiconductor memory device has the drawback that the scaling of gate length of a select gate transistor is more difficult than that of gate length of a memory cell transistor. The reason is as follows. A channel voltage that is cut off by a select gate transistor in write inhibit (or unwrite or unselect) mode does not drop due to the scaling. In other words, a boosted channel voltage cannot be cut off sufficiently if the gate length of a select gate transistor is decreased by scaling. The nonscaling of the select gate transistor is becoming a limiting factor in reducing the device in area.
In the future, the gate length of a select gate transistor needs to be decreased by scaling to meet a demand that a device should be reduced in area. A search for the best way to do so is being carried out.